Raoult’s Law and Its Application to Sublimation Vapor Pressures of Mixtures of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous pollutants resulting from incomplete combustion and many fuel processing and storage operations. They are encountered in the environment in many forms, ranging from non-aqueous phase liquid and/or solid mixtures (i.e., NAPL and NAPS materials), to dissolved aqueous phase components, to adsorbed species on solid surfaces. The present work concerns the vapor pressure behavior of non-aqueous phase mixtures of PAH, as may characterize tars or solid PAH phase contaminants. In particular, model mixtures of anthracene, perylene, and fluoranthene are examined herein. Mixture vapor pressure data are important to understanding and modeling exposure opportunities. There exist few data in the literature concerning the vapor-liquid and vapor-solid equilibrium behavior of mixtures of high molecular weight organic compounds as they tend to degrade at the high temperatures required to conveniently perform such measurements. This degradation issue is overcome by implementing the Knudsen effusion technique, an indirect method of vapor pressure measurement at temperatures sufficiently low to ensure thermal stability of the PAH studied. Our results indicate that commonly employed Raoult’s law assumptions may represent an over-simplification of some actual near-ambient PAH mixture behavior for the compounds studied, yet there are many instances in which this law is closely followed.